SYNTHESIS AND STRUCTURAL ASPECTS OF PHOSPHANE - BORANES DERIVED FROM TRIS(DIPHENYLPHOSPHINO)METHANE, HC(PPH2)3

The structure of tris(diphenylphosphino)methane, HC(PPh2)3 (1), has been determined by X-ray diffraction. The molecules have approximate C3 symmetry with the lone pairs at the three phosphorus atoms in an "all-trans" orientation. Variable-temperature NMR experiments (+35 to -90-degrees-C in CH2Cl2) show free intramolecular motion in solution. The partially or fully borylated adducts HC(PPh2)2[PPh2(BH3)] (2), HC(PPh2)[PPh2(BH3)]2 (3), and HC[PPh2(BH3)]3 (4) have been prepared from 1 and THF . BH3 or Me2S . BH3. For 2 and 3 the internal rotation about the phosphorus-methine carbon bonds is also essentially unhindered, but has been shown to be restricted in 4 (E(a) = 31.2 kJ/mol). The chemical shifts of the methine protons in these compounds can be calculated on the basis of a BH3 substituent constant.